1. Field of the Invention
The present invention relates to interferometry and, in particular, to high efficiency interferometers that can be employed in non-invasive optical imaging and measuring devices such as optical coherence tomography and optical coherence reflectometry.
2. Discussion of Related Art
Low coherence interferometry, which derives from classical white light interferometer, has received increasing scrutiny over the last decade or so for its application to optical coherence reflectometry and optical coherence tomography. Optical coherence reflectometry and optical coherence tomography are both techniques for mapping images of eyes and can be highly beneficial for diagnosing and curing defects in eyes. Further, low coherence interferometry can be utilized in endoscopy, laparoscopy, microscopy, and any other technique where interferometric techniques may be useful.
FIG. 1 illustrates an example of a conventional low coherence interferometer 100. Low coherence interferometer 100, as shown in FIG. 1, is a simple Michaelson interferometer that includes a light source 101, a beam splitter 102, and an optical signal processing unit 110. As shown in FIG. 1, beamsplitter 102 can be a 2×2 beamsplitter that splits a low coherence light beam from light source 101 received from source arm 103 into a reference beam coupled into reference arm 104 and a sample beam coupled into sample arm 105. The reference beam on reference arm 104 is reflected back to beam splitter 102 by reference 112, and the sample beam on sample arm 105 is reflected back to beam splitter 102 by sample 111. Beam splitter 102 splits the reflected reference beam into source arm 103 and signal arm 106. Similarly, beam splitter 102 splits the reflected beam from sample 111 into both source arm 103 and signal arm 106. The reflected light beam from sample 111 and from reference 112 are, therefore, combined into a combined beam coupled into source arm 103 by beam splitter 102. The signal beam in signal arm 106 is received by a photo-detector and a transmittance amplifier TIA 107, where the optical signal is converted to an electronic signal. The electronic signal is coupled into optical signal processing unit 110 for further processing. The function of the optical signal processing undertaken in optical signal processing 110 can include bandpass filtering, signal amplification, demodulation, lowpass filtering and other processing functions. The optical signals obtained at optical signal processing unit 110 can be processed either through hardware or software for imaging and analyzing the structure and optical properties of sample 111 (the sample under test).
An example of an optical coherence domain reflectometer based on the Michaelson interferometer as shown in FIG. 1 has been discussed by Youngquist & Davis in Optics Letter 12, 158-160, March 1987. An optical reflectometry with a transverse scan mechanism for tomographic imaging has been described by Park in Applied Optics 1987. Optical coherence tomography for imaging bio-tissue based on interferometry for bio-tissue image is also discussed in U.S. Pat. No. 5,321,501.
In the example of the Michaelson interferometer shown in FIG. 1, portions of the reflected signal from sample arm 105 and reference arm 104 also propagate into source arm 103. This is disadvantageous for optical performance. First, useful signal is lost to source arm 103. Second, the reflected light in source arm 103 will increase the noise on the light beam generated by light source 101.
A solution to reflected light into source arm 103 is described in Rollin's paper in Optics Letters Vol 24, No. 21, November 1999. As shown in FIG. 2, an optical circulator 202 is inserted in source arm 103 of interferometer 200. Beam path 203 is optically coupled between beam splitter 102 and circulator 202. As shown in FIG. 2, light reflected down beam path 203 from beam splitter 102 enters circulator 202 and is routed to detector 207 through beam path 205. The output signals from detectors 107 and 207 are combined in differential amplifier 208 and then input to optical signal processing unit 110. Such an arrangement serves two purposes: First, the reflected beams are routed into detector beam path 205 and 106; and second, circulator 202 also functions as an isolator to keep reflected light away from light source 101.
Another method is disclosed in U.S. Pat. No. 6,501,551. In the solution described in the '551 patent, both sample arm 105 and reference arm 104 include an optical circulator. The reflected signal from sample 111 and reference 112 are then routed to another beamsplitter that is different from beamsplitter 102. The two output signals of the new beamsplitter can be individually received, demodulated, and processed before one channel is subtracted from the other in a balanced detection receiver.
However, the signal strength measured at detector 107 is also sensitive to the polarization state of the light beam reflected from sample 111. It is particularly disadvantageous when the sample material of sample 111 is highly birefrigent. FIG. 3 illustrates an example interferometer system 300, as disclosed by Sorin in U.S. Pat. No. 5,202,745. Interferometer system 300 can be independent of the polarization state of the sample beam reflected from sample 111 because detector arm 106 can be optically coupled to a polarization diversity receiver. As shown in FIG. 3, the polarization diversity receiver can include polarization beamsplitter 305 coupled to optical detectors 310 and 311 by transmission arms 306 and 307, respectively. As shown in FIG. 3, light source 101 is first linear polarized by coupling light from source arm 103 into a linear polarizer 302. Polarization beamsplitter (PBS) 305 is placed in the detector arm 106 to split the beam into two orthogonally polarized beam paths 306 and 307. A polarization controller 308 can also be coupled into reference arm 104 and adjusted to produce equal reference signal power in each of polarization arms 306 and 307 of the polarization diversity receiver. These two polarization arm signals are individually demodulated and processed before being summed in optical signal processing unit 110. In the example interferometer system 300, no matter what the polarization state of the refelected beam in sample arm 105, the reflected beam from sample arm 105 will eventually interfere with its own properly polarized reference beam and the resulting beam will be summed. The signal is constant with respect to the changes of the polarization state of the sample beam.
However, neither of the systems illustrated in FIGS. 2 and 3 solve both the polarization problem and the problem of light reflected back into the light source. In light of above mentioned disadvantages of the prior art and other shortcomings, there is a need to resolve both polarization and light reflected into source arm issues in a single interferometer, as it is desirable to maximum the signal, consistent with a change of polarization due to the sample, to the detectors as well as to reduce the noise level from the light source.